Turn `io.spine.logging.MetadataKey` into abstract class

[alexander-yevsyukov]

Now it's interface with the direct implementation in io.spine.logging.jvm.MetadataKey class. Presumably, the class could be made platform-neutral.

The dependency the MetadataKey class has is io.spine.logging.jvm.backend.Platform. This could be solved using expect/actual mechanism for the Platform class.